Because wafers with silicon on thin buried oxide layers (BOX) or on ultra thin buried oxide (UTBOX) layers are advantageously characterized by small variations of threshold-voltages, they are of growing interest in present and future CMOS technology. In particular, fully depleted CMOS technology allows for low-voltage and low-power circuits operating at high speeds. Moreover, fully depleted silicon-on-insulator (SOI) devices are considered as some of the most promising candidates for reducing short channel effects (SCE).
Silicon on Insulator (SOI) wafers and, in particular, UTBOX wafers, can form the basis for high-performance MOSFET and CMOS technologies. The control of SCE is mainly facilitated by the thinness of the active silicon layer formed above the insulator, i.e. above the buried oxide (BOX) layer. In order to reduce the coupling effect between source and drain and, further, to improve the scalability of thin film devices for future technologies, very thin BOX layers are mandatory. Control of threshold voltages also depends of the thinness of the BOX layers. Appropriate implantation of the substrate below the BOX layer allows for accurate adjustment of the threshold voltage by back biasing.
In the manufacture of SOI devices, in particular, of CMOS devices, trenches usually have to be formed, for example, in order to electrically isolate individual MOSFETs from each other. The formed trenches can be cleaned, slightly oxidized, and subsequently filled with an oxide liner and some insulator material above the liner, so as to form shallow trench isolation (STI) structures. However, in the manufacture of such trench structures, for example, STIs, a problem known as the so-called Bird's Beak Effect can arise. The Bird's Beak Effect in the context of the formation of trenches in SOI wafers is characterized by the increase in the thickness of the BOX layer in the areas close to the edges of a trench.
One of the reasons behind the Bird's Beak Effect is related to under-etching of the BOX layer, as can, for example, occur during a cleaning process that usually comprises hydrofluoric dipping. Another reason behind the Bird's Beak Effect is lateral oxidation of a not completely closed bonding interface within the BOX layer. Such bonding layers with interfaces can arise when the SOI wafer is manufactured by oxide-oxide bonding wherein, for example, a thin silicon layer is covered by an oxide layer, for example, an SiO2 layer, and transferred to a substrate that is also covered by an oxide layer of the same type.
Thus, there is a need for improved methods for the formation of trenches and corresponding isolator structures that alleviates the Bird's Beak Effect.